Hi Everyone.
This is going to be quite a long post, but I find it very interesting, and I’ll be happy if others can do the same and post the results here.

Gordon, maybe it’s a good idea to add a “Comparison” forum for these things, there is no appropriate forum right now.

So, I decided to upgrade my lens from Tamron 18-200 to 2 high level Canon lenses. The first one was the 17-40L, the next one (when I have the money) will probably be the 70-200L f/4 IS.
I also have standard Canon lenses from my old film SLR (EOS 3000N): 28-80 and 75-300 III.

I decided to do a “lens test“ between the Tamron, the Canon 17-40 and the Canon 75-300.
I Placed the camera (EOS 350D) on a tripod, midday, ISO 100, and fine large JPEG.
I took photos in max aperture, f/8 and f/11 in each focal length. Using f/8 gave me better results than the max aperture (obviously), so I post here only the f/8 results.
All pictures are 100% cropped, no editing whatsoever.

Here are the results:
17-40 at 17:

18-200 at 18:

17-40 at 40:

18-200 at 42:

75-300 at 75:

18-200 at 76:

75-300 at 205:

18-200 at 200:

Strangely, it seems that at 17/18, the Tamron is sharper than the Canon L ( , can anyone explain this???).
At all other shots, the Canon lenses performed better.

Hi Liron, that's a good test - but from which part of the image is the crop taken from?

You're right about the Canon 17-40mm - when I tested one, it was super sharp at 17mm, but quite soft at 40mm towards the edges, although I have since heard of models exhibiting the opposite - so maybe the design was changed...

That's from the center? Crikey, your result for the 17-40mm at 17mm is a bit disappointing. I wouldn't have been too surprised if you'd said the edge, but I'd hope for better from the middle - maybe you got a bad copy - can you try another out?

This is all great stuff and something I'd recommend any enthusiast does for their lenses. All lenses have a sweetspot aperture where they perform best of all, and if you're after the ultimate sharpness, it's worth finding it. Do note though that this sweetspot could change with focusing distance...